The present invention relates a process for curing epoxy resin compositions. More particularly, it relates to the curing of polyepoxides with carboxylic acids or acid anhydrides using a diphenyliodonium salt accelerator to reduce curing time of the reaction.
The curing of epoxy resin composition with carboxylic acids or acid anhydrides is known. Generally a Bronsted acid may be used to produce homopolymerization and copolymerization of polyepoxides simultaneously. Useful Bronsted acid type curing agents are polyfunctional phenols, polyfunctional organic acids and anhydrides of mono and difunctional organic acids. These acid type curing agents react with polyepoxides in two different reactions as follows:
(a) first by an esterification reaction, ##STR1## (b) by an etherification reaction, ##STR2## PA1 c=d-e e=valence of M and is an integer equal to 2 to 7 inclusive, and PA1 d is &gt;e and is an integer having a value up to 8.
The esterification reaction may be catalyzed by either acid or base. When acid is used, the etherification reaction proceeds more rapidly. In a base the etherification reaction is suppressed as follows: ##STR3##
If a cyclic anhydride curing agent is used, the anhydride must first be converted to the monoester/monoacid in order to react as follows: ##STR4## The new acid reacts to produce a new hydroxyl group which reacts with a new anhydride group and so on.
The carboxylic acids and acid anhydride curing agents have a serious shortcoming in that they are inactive to cure polyepoxides at room temperatures. It is only at very high temperatures that acids or acid anhydrides become effective curing agents. This prevents their use in preparation of compositions that are to be cured at a lower temperature or compositions that might be injured by the high temperatures. Even at high reaction temperatures, the acids or acid anhydrides usually react very slowly so that they are generally undesirable for use when the cure must take place rapidly.
Accordingly, it has been proposed to add certain materials to accelerate the cure of epoxides with acids or acid anhydrides. One frequently employed additive is a strong Lewis acid. The Lewis acids suitable for catalyzing the polymerization of epoxy resins are those with unfilled orbitals in their outer shells, i.e. Friedel-Crafts type, such as stannic chloride or boron trifluoride. These Lewis acids are generally either gases or solids. Boron trifluoride, or gas, is useful for synthesizing aliphatic epoxies from epichlorohydrin but not so useful as a curing agent per se. The solid Lewis acids, like stannic chloride, are useful only in solution. These strong Lewis acids are extremely reactive, even with the glycidyl ether resins, in that the reaction progresses beyond the gel stage in from 30 to 90 seconds. The concentration of the catalyst is critical, above the critical concentration the reaction goes extremely rapidly, below, only gel structure results. Strong Lewis acids are inherently too fast for all but a limited number of commerical applications.
Another proposed additive is a phosphonium halide catalyst which is described by Dante et al. in U.S. Pat. No. 3,547,885. The phosphonium halides are effective at reducing the temperatures at which acids or acid anhydrides become active curing agents for polyepoxides from above 200.degree. C., generally to temperature of from 50.degree. C. to 200.degree. C., and usually between 100.degree. to 200.degree. C. These compositions still require heating to effect the curing of polyepoxides. When the compositions are used as coatings for electrical equipment, the heating required to activate the acid or anhydride curing reaction may also lead to material run off and loss prior to cure.
It has now been discovered that the diphenyliodonium salts may serve as outstanding accelerators in a curing reaction of polyepoxides with acids or acid anhydrides. Exhibiting sensativity to both ultra violet light and to heat, the salts are easily decomposed to produce Lewis acid catalysts for promoting the curing reaction. The curing reactions occur at much faster rates and at lower temperatures. An added advantage of these diphenyliodonium salts is that they may serve as photoinitiators for the curing reaction without the need for heating. They are especially useful in vacuum pressure impregnation systems and as coatings because they provide a choice of thermal or ultraviolet cure or a combination of both. For example, a coated article or vacuum pressure impregnated piece of equipment may first be irradiated with ultraviolet light to cocoon the article or mold at room temperature, followed by heating to fully, thermally cure the interior so that material run off or loss during the process may be minimized.